1. Field of the Invention
This invention relates to a process for the conversion of a gaseous paraffinic feed containing propane to liquid aromatics in the presence of a crystalline zeolite catalyst. More particularly the invention relates to processes for converting ethane and propane streams to aromatics and other liquid hydrocarbons.
2. Description of the Prior Art
Zeolites and alumina have been used in the past in the preparation of catalysts for the production of aromatic hydrocarbons from open chain hydrocarbons. The open chain hydrocarbon is passed over the catalyst at an elevated temperature in the liquid or vapor phase. Zeolites of various types, particularly those containing a high silica-to-alumina ratio, have been suggested for the preparation of such catalysts. Examples of such zeolites are mordenite and the ZSM varieties. Such zeolites have been known to contain gallium in the form of its oxide which is substituted either partially or wholly for the aluminium oxide present therein. These zeolites however, contain gallium as part of the crystal structure of the zeolite and the gallium is non-ionic. However, the yields of aromatic hydrocarbons from such open chain hydrocarbons have been unsatisfactory when using catalysts prepared from such zeolites.
U.S. Pat. No. 4,180,689 teaches that by using catalysts which contain gallium and which are prepared from specific types of aluminosilicates improved yields of aromatic hydrocarbons may be obtained if gallium-containing catalysts are prepared from specific types of aluminosilicates. This patent further discloses that if the gallium is either exchanged for one of the cations or protons or impregnated into the zeolitic cavities surprisingly high catalytic activity is obtained especially in hydrocarbon conversion process. The feedstock for this process are C.sub.3 -C.sub.12 feedstock of either a single component or mixtures of saturated and unsaturated hydrocarbons.
U.S. Pat. No. 4,120,910 discloses that aromatic compounds can be produced by contacting, in the absence of added air or oxygen under suitable conversion conditions a gaseous, paraffinic feed stock containing a high percentage of ethane with a ZSM-5 type crystalline aluminosilicate zeolite catalyst having incorporated therein a minor amount of a metal or metal oxide from Group VIII, IIB, or IB. Especially preferred is a zinc-copper mixture.
U.S. Pat. No. 4,097,367 teaches the catalytic conversion of olefinic naphthas which contain diolefins over a special catalyst to yield a product stream which contains little or no non-aromatics boiling in the range of benzene, toluene and xylene. The catalyst is a combination of zinc and a metal from Groups IB and VIII of the Periodic Table with a crystalline aluminosilicate zeolite having a silica-alumina ratio greater than 12 and a constraint index not less than one nor greater than 12.
A problem associated with the use of zeolites which contain zinc has been the loss of zinc from the catalyst when gases are flowed through a stationary bed of catalyst or through a fluidized catalyst system. At the high temperatures necessary for the reaction, for example, the conversion of ethane and propane to aromatics, the vapor-pressure of the zinc becomes such that the zinc eventually is eluted from the catalyst. Consequently, catalyst activity is lost in a matter of days or hours under the reducing atmosphere of the hydrocarbon charge.
U.S. Pat. No. 4,097,367 teaches that metals, such as palladium, can be composited with the zinc on the zeolite catalyst and the elution of zinc can be retarded or prevented. The palladium does not enhance the activity of the catalyst except that when combined with zinc on ZSM-5 zeolite palladium is seen to improve selectivity of the catalyst for production of desired lower boiling aromatics.
The cost of the palladium is such, however, that the development of less expensive metal stabilizer is desirable.
We have now discovered that incorporating gallium in place of palladium into a zinc zeolite catalyst results in a more stable catalyst (zinc is eluted at a slower rate). Less concentration of gallium is required and substantial cost savings can be effected.